The estimation of cotton-growing areas by Remote Sensing
Spectrum determination of cotton and the corresponding crops
Different growth and development steps of crop-coenosium have different tones. the way that the crops absorb and reflect spectrum are different. it is the basis of estimation of cotton-growing areas to choose the phonological periods which the colony's tones and compositions are most different from the other crops. In the test area, the crops have the same periods of growth and development as cotton are soybean, wheat, peanut, sweat potato and maize, etc. (called the corresponding crops). Therefore, we should pay special attention to seek maximum differences of spectrum of growth and development periods between cotton and other crops.
In 1987, the research group used the objects spectrometer to determine the spectral reflectance of cotton, the corresponding crops i.e. soybean, peanut, sweat potato and maize, and the exposed soil in periods of cotton's heading (July 4), end of budding and beginning of blossom (July 23), flowering and bulling (August 1), yield formation (August 25), initial and middle periods of ripening (Sept. 29) middle and late periods of ripening (October 10). We have found that, in the periods of cotton's heading, initial and middle reopening, the difference of characteristic curve of spectrum between the cotton fields and the other crops' is evident (Figure 1.)
Figure 1. The characteristics curves of spectrum of cotton and the
corresponding crops in each of the growing and developing period in 1987
In 1988, the research group, separately determined the spectral reflectance of cotton and the corresponding crops in the periods of cotton's establishment (July 23), heading (July 10), yield formation (August 29), initial and middle periods of ripening (Sept. 28). parts of their characteristic curves of spectrum are as Figure 2.
Figure 2. The characteristics curves of spectrum of cotton and the
corresponding crops in each of the growing and developing period in 1988
The spectral data acquired in 1987 and 1988 showed that the heading period and initial and middle periods of ripening are the most favorable time to distinguish the cotton fields from other objects. it is unanimous with that, shown in table 1 of the development period of cotton and the corresponding crops, the difference between cotton and other crops in this two periods is the greatest.
The selection of Remote Sensing data
In order to get the higher accuracy of estimation, the TM digital image data of high resolution power should be selected n the cotton's heading period and initial and middle periods of ripening.
We selected the July 12,1986 TM digital images of Huojia country which was the heading period of local cotton. Band 2,3,4 of TM digital images which are the most favorable to the distinguishment of soil and vegetation are mainly used.
